This invention relates generally to DC motors and, more particularly, to a highly efficient, sealed, brushless, disk DC motor having inner and outer rings of permanent magnets mounted on cylindrical surfaces of outer and inner rotors, respectively, and having a ring of electromagnets mounted on a stator such that inwardly facing surfaces of the permanent magnets rotate in close proximity to adjacent opposite surfaces of the ring of electromagnets.
Variable speed electric motors of different types have been employed in a variety of applications over the years, most recently in electric vehicles. One such motor is the series DC motor, which employs brushes and wound coils. These motors suffer from low peak power densities on the order of 0.3 horsepower/pound. Thus, they are heavy and unresponsive, as a typical 40-horsepower motor weighs 130 pounds. They are not sealed, since air flow through the motor is required to cool the rotor, and they cannot run in reverse. Their speed is difficult to control under varying load conditions, and they have no power regeneration capability.
Three-phase AC induction motors are typically powered by a DC battery pack coupled to an DC/AC inverter and associated pulse width modulation circuitry to achieve variable speed control. These motors are characterized by heavy weight, low torque, and power factor losses.
Representative of prior art brushless DC motors are the High Power Density Brushless DC Motor described in U.S. Pat. No. 4,187,441; the Toroidally Wound Brushless DC Motor described in U.S. Pat. No. 4,547,713; and the Brushless Motors taught in U.S. Pat. Nos. 4,801,830; 4,982,125; 5,637,945; 5,689,147; and 5,747,910.
In accordance with the illustrated preferred embodiment of the present invention, a brushless DC motor employs a flat circular non-ferrous stator plate having a plurality of electromagnets mounted in a ring pattern on an inner face thereof. A plurality of permanent magnets are mounted in equal numbers in inner and outer ring patterns on the outer and inner cylindrical surfaces, respectively, of a pair of steel rotors of different diameter that rotate in concert. The stator plate and the pair of rotors are axially and diametrically aligned such that the inner ring of permanent magnets rotates in close proximity to and inside the ring of electromagnets, and the outer ring of permanent magnets rotates in close proximity to and outside the ring of electromagnets. The electromagnets utilize tape-wound amorphous metal cores to minimize eddy currents and resultant heat losses and to permit the use of heavier gauge copper windings to minimize resistive power losses and attendant heat. The DC motor of the present invention is capable of accommodating a larger number of poles, in the form of permanent magnets, than prior art brushless DC motors, the number being limited only by the diameter of the rotor, thus providing significantly increased power and torque. The precise location of the rotor is monitored through the use of three Hall effect sensors, and conventional three-phase pulse width modulation (PWM) control circuitry is employed as a source of operating power and to control the speed of the motor. The present motor exhibits higher power density and lighter weight than prior art brushless DC motors. It can be driven to 200% of its current rating to thereby double the output horsepower for short periods of time. It is reversible, is capable of power regeneration, and offers good speed control. By employing a large diameter rotor and many electromagnet pole pieces, the present motor exhibits up to 200% more starting torque than conventional brushless DC motors, thus eliminating the need for a gear box or clutch in electric vehicle applications. Since the rotor components are individually assembled and the electromagnet coils can be machine wound, the present motor can be manufactured at a cost saving over conventional AC induction motors.